1. Field of the Invention
The present invention relates to an image pickup lens system and particularly, to an image pickup lens system of a single lens construction, which is used in an image pickup device (e.g., a CCD camera) utilizing an image pickup element such as CCD, CMOS and the like mounted in a mobile telephone or the like, and which can be reduced in weight and size.
2. Description of the Related Art
In recent years, the multimedia have been developed remarkably, and for example, the demand for a camera utilizing an image pickup element such as CCD, CMOS and the like adapted to be mounted in a portable computer, a visual telephone, a mobile telephone and the like, e.g., a CCD camera, is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight.
Therefore, it is also desired that an image pickup lens system used in such a CCD camera is likewise small in size and lightweight.
A single-lens system using a single lens is conventionally used as such an image pickup lens system.
There is such a conventionally known image-pickup lens systems of a single lens type disclosed, for example, in Japanese Patent Application Laid-open No.10-282410 and the like.
A ¼ inch sensor (having a diagonal dimension of about 4.5 mm) has been conventionally used in many cases as a solid image pickup element mounted in a mobile telephone. In recent years, however, a 1/7 inch sensor (having a diagonal dimension of about 2.6 mm) of a further reduced size has been used mainly in place of the above-described ¼ inch sensor.
In the image pickup lens system of such an extremely small size, it is necessary to meet the following conditions:
First of all, in the image pickup element such as CCD, CMOS and the like, 100% of light obliquely entering into the image pickup element cannot be utilized for the structural reason, unlike a silver-salt film and hence, it is required that light enters into a lens at an angle closer to the vertical at any position on the image pickup element. In other words, it is required that a telecentric property is high, and a distance between the image pickup element and a pupil is large.
In the image pickup element, light is sensed by each of picture elements provided in the image pickup element, but the sensitiveness of each picture element within the image pickup element is uniform and hence, it is desirable that the amount of light entering into the image pickup element is kept constant at any position within the image pickup element. In other words, it is required that the amount of light is uniform in a central portion and a peripheral portion of the image pickup element and hence, it is required that the amount of light in the peripheral portion ensured as much as possible.
These conditions are more important than those in the prior art, because the size of each picture element is reduced with a reduction in size of the image pickup element and as a result, the sensitiveness is also reduced.
Further, in general, a cover glass is disposed on a surface of the image pickup element, and depending on the application, any one of various filters (such as an IR cutting filter, a low-band pass filter and the like) is also inserted. Therefore, a relatively long back focal length is required.
For example, in an image pickup lens of a wide-angle type used in the ¼ inch sensor, it is relatively easy to ensure a distance between the image pickup element and a pupil and a back focal length, because the focal length is as relatively long as about 4 mm.
In an image pickup lens of a wide-angle type used in the 1/7 inch sensor, however, the focal length is as extremely short as about 2 mm. For this reason, the distance between the image pickup element and a pupil and the back focal length are also shortened with a reduction in focal length. Therefore, the following problem is encountered: It is extremely difficult that the image pickup lens used in the conventional ¼ inch sensor is reduced in size and used for the /7 inch sensor. When the size of the lens is merely reduced, as described above, there is also a problem that the accuracies of the thickness of the central portion of the lens and the thickness of a flange are degraded remarkably.
The position of the pupil can be set at the maximum by disposing a diaphragm in the vicinity of the position of the focal length on the side of an object. In this case, however, the following problem is encountered: The size of the entire lens (the thickness and the effective diameter of an optical face) is increased extremely, but also the workability of the lens is degraded.
Further, the back focal length can be improved by forming the lens into a meniscus shape, but there is a problem that if the lens is not of an appropriate shape, the workability of the lens is degraded.